Is Nizwa worth staying in overnight?
Yes, Nizwa is worth an overnight if you want more than a quick fort photo. It makes inland Oman easier: souqs, villages, forts and mountain roads sit better from here than from a distant Muscat hotel.
Answer
Nizwa gives the trip a useful change of gear. The town itself has enough atmosphere for a wander, but its bigger value is geographic: it puts you closer to the inland forts, old villages and highlands that make Oman feel different from a Muscat-based break. A day trip can work if time is tight. Staying overnight is better if you want an early start, a less rushed souq visit or a mountain day that does not begin and end with a long return drive. It is not glamorous; it is practical in the best way.